Alkali and alkaline-earth metal dodecahydro-closo-dodecaborates: Probing structural variations via neutron vibrational spectroscopy

The hydrogen-weighted phonon densities of states for the series of alkali (A = Li, Na, K, Rb, and Cs) and alkaline-earth (Ae = Mg, Ca, Sr, and Ba) metal dodecahydro-closo-dodecaborates, A2B12H12 and AeB12H12, were measured via neutron vibrational spectroscopy (NVS). Using the known crystal structures, density functional theory (DFT) phonon calculations were able to closely replicate the observed vibrational spectra. The spectral details were found to differ considerably with structure, indicating that the internal vibrations of the B12H122- icosahedral anions are sensitive to symmetry-dependent interactions with their crystal surroundings. In contrast, these internal vibrations were relatively unchanged among isomorphic A2B12H12 and AeB12H12 compounds possessing different metal cations. These results confirm that the combination of NVS and DFT phonon calculations can be used to help validate postulated local crystal symmetries in these types of materials, even in instances where the ordering is only short-range, rendering the materials amorphous with respect to diffraction probes.